Microwave circulator

ABSTRACT

Microwave circulator can be prepared by depositing magnetic coatings using a direct current arc-plasma generator. The magnetic coatings can be garnets or ferrite compounds. The devices are prepared by first forming a depression in the surface of a dielectric material, and then subsequently depositing the magnetic material using arc-plasma spraying to fill the depression. The invention further contemplates the additional steps of removing excess magnetic material and providing a coplanar surface which may subsequently be metallized.

United States Patent Harris et a1.

MICROWAVE CIRCULATOR Inventors: Douglas H. Harris; Harold M. Correll, both of Dayton, Ohio Assignee: Monsanto Research Corporation, St.

Louis, Mo.

Filed: Apr. 30, 1973 Appl. No.: 356,255

Related US. Application Data Division of Ser. No. 205,875, Dec. 8, 1971.

US. Cl. 117/212, 117/46 FS, 117/46 FB, l17/93.l PF, 117/213, 117/217, 117/235. 117/236, 117/239 Int. Cl. H011 10/04 Field of Search 117/235-239, 117/240, 93.1 PF, 46 F8, 46 F8, 212, 213, 217, 236

References Cited UNITED STATES PATENTS 2/1963 Brownlow 117/235 X [4 1 Dec. 17, 1974 3,148,263 9/1964 Jensen 219/75 3,183,337 5/1965 Winzeler 3,313,908 4/1967 Unger et al. 3,356,976 12/1967 Sampson et a1... 3,576,672 4/1971 Harris et al. 340/174 X Primary ExaminerWilliam D. Martin Assistant Examiner-Bernard D. Pianalto Attorney, Agent, or FirmBruce Stevens [57] ABSTRACT Microwave circulator can be prepared by depositing magnetic coatings using a direct current arc-plasma generator. The magnetic coatings can be garnets or ferrite compounds. The devices are prepared by first forming a depression in the surface of a dielectric material and then subsequently depositing the magnetic material using arc-plasma spraying to fill the depression. The invention further contemplates the additional steps of removing excess magnetic material and providing a coplanar surface which may subsequently be metallized.

4 Claims, 3 Drawing Figures PATENTED DEC] 7 I974 SHEEY 2 BF 2 MICROWAVE CIRCULATOR This is a division of application Ser. No. 205,875, filed Dec. 8, 1971.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to a process for depositing magnetic materials in a depression on a dielectric substrate. The depressions are filled by projecting heated, particulate, magnetic materials onto the substrate using an arc-plasma torch.

2. Description of the Prior Art Integrated microwave circuits have existed in various forms for many years. These circuits have the advantages of being light weight, compact and compatible with designs of microwave power and signal control modules. 7

One of the techniques that has been the subject of much effort is the use of thin ferrite substrates as the basis for microwave integrated circuits. The term ferrite" as used here refers to those ferromagnetic compounds containing Fe O The term is used in the language of commerce to refer to compounds containing Fe O as for example, barium ferrite, BaFe O nickel zinc ferrite, e.g., Ni Zn Fe O,, or those forms of garnet structures such as Y Fe O, etc. It should be noted that the term ferrite as used here does not refer to the allotropic forms of metallic iron, sometimes designated a-ferrite and B-ferrite.

Ferrites are commonly prepared by mixing together the constituent metal oxides, carbonates, or oxalates and prefiring them at about l,000 C. Then, they are ground and mixed again, and either applied as a coating or pressed into a shaped object and sintered at l,l00 to l,450 C. The final fired object is polycrystalline and usually is somewhat porous. Depending upon their composition, they can have any of several crystal structures, such as spinel, magneto-plumbite, distorted perovskite or garnet. Their composition can vary considerably from one ferrite to another as long as there is one or more metal in combination with Fe O as for instance BaO in barium ferrite or MnO, ZnO and MgO in a manganese-zinc-magnesium ferrite. A portion of the Fe- O may be replaced by the oxide of metal in the third oxidation state, as for example aluminum oxide, A1 0 Coatings of ferrites have been made in the past by ceramic techniques, often with considerable difficulty in view of the problems encountered. Thus, slips or slurries of ferrites and water have been applied to the surface of plates or drums, then dried and subsequently baked to harden. Often the coatings have been nonuniform and discontinuous due to shrinkage and cracking. The high temperatures required for sintering, usually over l,000 C., have caused warping of the coated substrate as it has cooled, due to the differences in the coefficient of thermal expansion of the ferrite and of the metallic substrate.

A particularly advantageous method of depositing ferrites on a substrate was taught in U.S. Pat. No. 3,576,672 issued Apr. 27, 197] to Douglas H. Harris and Richard J. .lanowiecki. This patent teaches a method of depositing a ferrite on a substrate which comprises introducing a finely powdered ferrite into a powder carrier gas, passing the gas and the ferrite through a direct current arc-plasma gas stream so as to heat the ferrite, and causing the heated ferrite to impinge against and adhere to a substrate. The process as taught in U.S. Pat. No. 3,576,672 is used in the present invention and that process is incorporated herewith by reference.

Conventional microwave integrated circuits such as microstrip microwave circulators, can use total ferrite substrates. The method is bothered by higher loss since metallization losses on ferrite are about twice the value achievable on nonmagnetic dielectrics. Since only a fraction of the ferrite area is necessary for device purposes, another approach is to insert a ferrite piece into a hole that has been specifically cut in a non-magnetic dielectric for that purpose. The latter method suffers from a number of disadvantages. The glue interface between the puck and the dielectric creates high losses and degradation of the efficiency of the device. Furthermore, the pucks and holes are difficult to fabricate with the precision required. The interface between the ferrite and dielectric is often irregular and results in metallization discontinuities.

Previously, ferrites have been sprayed or sputtered from the gaseous phase as disclosed in U.S. Pat. No. 3,100,295 issued Aug. 6, 1963 to Schweizerhof. Ferrites sprayed with a combustion-type flame spray gun are generally unsatisfactory, however, due to limited conditions of temperature and enveloping atmospheres as well as inadequate process control, which results in low quality crystalline and magnetic properties. Thus, the flame-sprayed coating may have poor stoichiometry, non-uniformity, low density, etc. Furthermore, they may be contaminated by undesirable phases or by the by-products of the combustion of the flame, i.e., oxides of carbon and/or water vapor. Sputtering generally requires bulky evacuated chambers and heated sources, and is a slow and tedious method of deposition, often resulting in unsatisfactory stoichiometry control in many oxide compounds.

SUMMARY OF THE INVENTION The object of this invention is to make co-planar composite substrates for microwave devices.

This object is achieved by a method which comprises the steps of forming a depression in a surface of a dielectric substrate, and depositing a magnetic material into the depression using arc-plasma spraying to fill the depression. In a more limited embodiment the invention also contemplates the additional step of removing excess magnetic material from the surface of the substrate. As another limited embodiment the invention further contemplates the additional step of metallizing the surface after excess magnetic material and dielectric substrate have been removed from the surface of the composite substrate.

The utilization of activated species, ions and high temperatures resulting from an electric discharge in a gas to produce a plasma is well-known in the art. Although the direct current plasma has gained wide commercial acceptance for spray coating applications, the radio-frequency induction plasma can be used to deposit materials onto substrates. The lack of electrode contamination in the radio-frequency plasma results in sprayed deposits of extreme purity, and the technique is widely used in the fabrication of high-purity crystals. Although the direct current arc-plasma torch was used herein, it will become apparent to those skilled in the art that a suitable radio-frequency induced plasma gas stream could be used in the practice of this invention.

One embodiment of a direct current plasma gas stream may be an arc-plasma torch such as any one of several commercially available. It may also be a torch especially designed to provide some specific processing advantage, e.g., more satisfactory powder feeding, use of special arc gases to realize temperature or environmental advantages, etc. providing the torch employs an electric direct current are to produce a hot stream of dissociated and/or ionized gas. A typical plasma spray gun and accessory equipment is shown in Plasma Jet Technology, NASA SP-5033, October, 1965, page 44. An improved version is shown in Bulletin No. 178 published by Metco, Inc., Westbury, NY. Another useful torch is that describedin U.S. Pat. No. 3,183,337, issued May Il, 1965 to Winzeler et al. Still another torch is that described in US. Pat. No. 3,148,263, issued Sept. 8, I964 to-Jensen. The electric power supply may be controlled both as to voltage and current, higher temperatures in the plasma effluent usually resulting from higher wattage inputs to the electrodes. The plasma arc gas may be nitrogen, helium or argon and mixtures thereof, nitrogen or argon mixed with hydrogen, or even air or oxygen. The desired stoichiometry of metal-to-oxygen in the ferrite can be maintained by control of oxygen in the plasma gas or powdercarrier gas or shroud gas or external gas, in addition to control of. plasma gas temperature and composition, residence time, and powder particle size. The torch input power, are gas flow rate and heat losses due to electrode cooling control the plasma stream enthalpy and hence the temperature at the torch nozzle exit. Plasma stream temperature, residence time and mode of powder injection affect melting of feed powder particles of a particular size. The powder-carrier gas generally employed in the past has been argon, but oxygen or oxygen-containing gas mixtures have also been used. The powder is conveniently fed into the are by a powdercarrier gas at a point near the nozzle exit. Finely divided powders, e.g., minus 44 microns, normally give better deposits because they can be heated quickly and uniformly to a softened state before being blown against the substrate. Although the material ultimately forming the deposited coating may be in a partially fluid state, it is usually not in a gaseous or vapor state.

For some ferrites an arc-plasma torch provided with a means of conducting a shroud gas to and around the heated ferrite, e.g., the torch described in U.S. Pat. No. 3,313,908 issued Apr. 11, 1967 to Unger et al, is convenient. The carrier gas and shroud gas is preferably oxygen for the production of satisfactory ferrite deposits. Thus plasma-deposited magnesium-manganesealuminum-substituted ferrites and nickel-zinc ferrites laid down using an oxygen shroud gas have markedly higher (superior) resistivity compared with deposits prepared under argon shroud gas.

The plasma deposition process can be used with a variety of substrates including even plastic or paper substrates which are heat sensitive. We have found, however, to make the co-planar composites of this invention that ceramic dielectrics are required. In particular the ferrites deposited by arc-plasma spraying require further processing on heated substrates and are annealed to promote grain growth; consequently, the substrates must have a coefficient of thermal expansion that is near, or matches, the coefficient of thermal expansion of the magnetic material. Examples of suitable dielectric substrates include magnesium titanate compounds and magnesia aluminate spinel mixtures with or without an excess of magnesia. Suitable magnetic materials would include ferrites such as magnesium manganese ferrite, yttrium iron garnet, lithium titanium ferrite, nickel zinc ferrite or modifications of the above materials.

Accordingly, our preferred process comprises forming a cavity, slot or other depression in a dielectric by suitable techniques, such as ultrasonic techniques, a diamond saw, etc. and filling the depression with ferrite using arc-plasma spraying techniques. The arc-plasma spraying process is the same as that taught in U.S. Pat.

No. 3,576,672 issued Apr. 27, 1971, which is herein incorporated by reference, and which comprises forming an arc-plasma stream from a flowing arc gas by means of a direct current are, introducing a finely powdered ferrite into a flowing powder carrier gas, passing the powder carrier gas and the ferrite into the arc-plasma stream so as to heat the ferrite, providing a shroud gas surrounding the ferrite, providing a preheated substrate at a distance from the torch nozzle of between 0.25 and 4.5 inches, and causing the heated ferrite to impinge against and adhere to the dielectric substrate.

The present invention also contemplates additional steps in the process for making suitable electrical devices. After the depressions in the substrate have been filled with the ferrite, the excess and overspray are removed by any number of techniques, such as the Free Abrasive Machining Process of the Specdfam Corporation, Des Plaines, Illinois, described in their Technical Bulletin No. 20-170. The resulting structures exhibit a good bond between the dielectric and the ferrite, and consequently, part of the dielectric can be removed from the surface of the device along with the overspray and excess ferrite to produce a device having the desired electrical characteristics. Further, the bottom layer of the dielectric can be removed to provide complete penetration of the ferrite through the composite. The device is then ready for metallization by techniques which are well-known in the art.

Examples of useful devices which are prepared by the present invention include a microwave limiter as in U.S. Pat. No. 3,356,967 issued to Honig, a circulator as in U.S. Pat. No. 3,355,679 issued to Carr, and a switchable circulator as in U.S. Pat. No. 3,355,680 issued to Saltzman et al.

BRIEF DESCRIPTION OF THE DRAWINGS Some of the novel devices which can be made by the process of the present invention are shown in the accompanying figures. In FIGS. 1 and 2 there is portrayed a circulator on a co-planar composite in which a ferrite island is inserted in a dielectric. FIG. 3 shows a coplanar approach to a ferrite limiter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is further illustrated by, but not limited to, the following examples:

EXAMPLE 1 This example describes a microwave ferrite circulator and a method of producing such a device.

In a MgTiO -type substrate, a product designated as D! 3 and manufactured by Trans-Tech, Inc., Gaithersburg, Maryland, a cavity was precision machined by ultrasonic techniques using a preformed tool. The cavity was approximately 0.025 inch deep and approximately 0.25 inch in diameter. A mask of thin nickel having a hole the same diameter as the cavity was then placed over the surface of the dielectric, and the cavity was filled with TT-l-l05, magnesium manganese ferrite with aluminum substitution (as described by Trans- Tech, Inc. in Ferrite Bulletin No. 105-67) by arcplasma spraying techniques as described in US. Pat. No. 3,576,672. After the cavity in the dielectric had been completely filled with the ferrite, the overspray of the ferrite on the dielectric and the excess ferrite were removed by the Free Abrasive Machining Process of the Speedfam Corporation, Des Plaines, Illinois, as described in their Technical Bulletin No. -170. The resulting structure appears as the co-planar substrate of FIG. 1, consisting of a ferrite island 11 in the magnesium titanate dielectric 12. When the article was annealed to increase grain size and appropriate metallization was placed over the co-planar substrate, a microwave ferrite circulator was produced having usefulv electrical properties.

EXAMPLE 2 The procedure of Example 1 was repeated. Then, the bottom layer of the dielectric was removed to provide complete penetration of the ferrite through the dielectric. The device is shown in FIG. 2 wherein the ferrite island 21 is shown in the dielectric 22. After the device was annealed to increase the grain size of the ferrite by techniques well-known in the prior art, metallizations are placed over each end of the ferrite island to produce a part having important electrical properties.

EXAMPLE 3 The process of Example I was repeated except that a gold strip was placed over the dielectric and ferrite after annealing to produce the device of FIG. 3. The dielectric substrate 31 contains a ferrite island 32. The device has a strip of gold 33 across the dielectric having a necked-down portion 34 across the top of ferrite 32. A gold ground plane 35 is added to the bottom side of the composite substrate. The device is a planar approach to a ferrite limiter and had useful electrical properties.

Although the invention has been described in terms of specified embodiments which are set forth in considerable detail, it should be understood that this is by way of illustration only and that the invention is not necessarily limited thereto, since alternative embodiments and operating techniques will become apparent to those skilled in the art in view of this disclosure. Accordingly, modifications are contemplated which can be made without departing from the spirit of the described invention.

We claim:

1. A microwave circulator comprising a co-planar substrate of a. a dielectric substrate having a cavity therein, and

ium titanium ferrite, and nickel zinc ferrite. 

1. A MICROWAVE CIRCULATOR COMPRISING CO-PLANAR SUBSTRATE OF A. A DIELECTRIC SUBSTRTE HAVING A CAVITY THEREIN, AND B. A MAGNETIC MATERIAL DEPOSITED BY ARC-PLASMA SPRAYING TECHNIQUES FILLING THE CAVITY, AND A METAL COATING ON SAID COPLANAR SUBSTRATE.
 2. A circulator of claim 1 wherein the coefficient of thermal expansion of the dielectric is near the coefficient of thermal expansion of the magnetic material.
 3. A circulator of claim 1 wherein the dielectric is selected from the group consisting of magnesium titanate, and magnesia-spinel.
 4. A circulator of claim 1 wherein the magnetic material is a ferrite selected from the group consisting of magnesium manganese ferrite, yttrium iron garnet, lithium titanium ferrite, and nickel zinc ferrite. 